Method for removing perfluorooctanoic acid in water by floc pre-deposition-ultrafiltration combined process

Abstract

The invention provides a method for removing perfluorooctanoic acid in water by a floc pre-deposition-ultrafiltration combined process. The method comprises the following steps: pre-depositing a ferric salt floc functional layer on the surface of an ultrafiltration membrane to form a composite separation system; and adding perfluorooctanoic acid with a certain concentration, standing and diffusing, and then carrying out ultrafiltration separation under the nitrogen low pressure of 0.1 MPa. According to the method, efficient removal of perfluorooctanoic acid is realized under low operating pressure, the removal rate can reach 57.6% at most under optimized conditions, and the method is obviously superior to a traditional ultrafiltration process. The method is simple and convenient to operate and low in energy consumption, is suitable for removing new perfluorinated pollutants in the fields of drinking water, industrial wastewater and the like, and has a good application prospect.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, specifically relating to a method for removing perfluorooctanoic acid (PFOA) from water using a combined floc pre-deposition and ultrafiltration process. Background Technology

[0002] Per- and polyfluoroalkyl substances (PFAS) are a class of highly stable synthetic organofluorine compounds. Perfluorooctanoic acid (PFOA), a typical example, is widely used in textiles, fire-fighting foams, and food packaging, and is frequently detected in groundwater and surface water in many countries and regions worldwide. The extremely strong CF bonds, hydrophobic and oleophobic properties, and environmental persistence of PFOA molecules make them difficult to remove effectively by traditional water treatment methods: coagulation, while a pretreatment technology, has limited removal efficiency when used alone; activated carbon adsorption is easily affected by water quality conditions (such as pH and competitive adsorption by organic matter), and regeneration costs are high; advanced oxidation technologies (such as photocatalysis and ozone oxidation) can effectively degrade PFOA, but they suffer from high energy consumption and the potential generation of toxic intermediates; and biological methods typically require specific environmental conditions, making large-scale application difficult in practice. Of particular concern is that PFOA not only has environmental persistence but can also accumulate in organisms through the food chain, posing a potential threat to ecosystems and human health. Therefore, developing efficient and economical PFOA removal technologies has become one of the key research focuses in the current water treatment field.

[0003] In recent years, membrane separation technology has been considered a promising new method for pollutant removal due to its advantages such as simple operation, small footprint, and no need for chemical reagents. Current research mainly relies on nanofiltration (NF) and reverse osmosis (RO) processes. While these processes show high rejection rates for new pollutants such as PFOA, they require maintaining high operating pressures, resulting in high energy consumption, high operating costs, and the discharge of concentrated wastewater containing pollutants. Therefore, exploring a process that combines high removal efficiency, low operating pressure, and economic feasibility is crucial.

[0004] Current technological status: Patent CN101928078A discloses a method for treating wastewater containing low concentrations of perfluorooctanoic acid (PFOA) using a membrane-based process. This technology pre-treats the wastewater with PFOA using ultrafiltration or microfiltration to remove suspended particles. Then, it uses a three-stage nanofiltration system to progressively concentrate the PFOA in the wastewater. Each nanofiltration stage employs an internal recirculation of the concentrate, ultimately concentrating the PFOA to a high concentration. The permeate can be directly discharged or reused, achieving PFOA recovery and zero wastewater discharge. However, this technology primarily targets the concentration and recovery of PFOA from industrial wastewater. The resulting high-concentration PFOA concentrate still requires further treatment (such as acidification and distillation) to truly realize resource recovery, increasing process complexity and treatment costs. Furthermore, this process has high requirements for influent water quality, necessitating pre-treatment to remove suspended particles, and the nanofiltration system operates at high pressure, raising concerns about energy consumption.

[0005] Patent CN106861445A discloses a low-pressure membrane water treatment technology based on a "sandwich"-style loose floc protective layer. This process uses an integrated membrane coagulation reactor, intermittently adding aluminum or iron salt coagulants to form a loose "sandwich" protective layer on the membrane surface through hydrolyzed flocs. The flocs then adsorb small-molecule organic matter such as humic acid from the water, effectively mitigating membrane fouling. However, this technology primarily targets the removal of conventional organic matter (such as humic acid), and its effectiveness in removing novel pollutants like PFOA, which possess strong chemical stability, high surface activity, and difficult adsorption characteristics, remains unknown.

[0006] Patent CN118788138A discloses a thin-film composite nanofiltration membrane and its preparation method for treating high-hardness water. The resulting nanofiltration membrane achieves highly efficient retention of PFOA and PFOS (perfluorooctane sulfonic acid). This invention constructs a sodium alginate polyelectrolyte interlayer on the surface of the ultrafiltration support membrane, and then forms a polyamide active layer through interfacial polymerization on this layer. By utilizing the interlayer to regulate the interfacial polymerization reaction, the pore size distribution and surface charge characteristics of the membrane are optimized, providing an effective means for the deep treatment of water sources containing new pollutants. However, this method has a complex preparation process, high operating pressure, and only physically retains pollutants on the concentrate side, failing to achieve harmless degradation, posing a risk of secondary pollution and high operating energy consumption. Summary of the Invention

[0007] This invention aims to overcome the shortcomings of existing technologies. It addresses the low removal efficiency of ultrafiltration alone for new perfluorinated pollutants (taking perfluorooctanoic acid, PFOA) and the problems of high energy consumption, large equipment investment, and difficult concentrated wastewater treatment in high-pressure membrane processes such as nanofiltration and reverse osmosis. The invention provides a method for removing perfluorooctanoic acid from water using a floc pre-deposition-ultrafiltration combined process that is low in operating pressure, simple to operate, and economical and efficient.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for removing perfluorooctanoic acid (PFOA) from water using a floc pre-deposition-ultrafiltration combined process. The method is characterized by pre-depositing an iron salt floc layer on the surface of an ultrafiltration membrane to construct a stable floc layer-ultrafiltration membrane composite system, achieving adsorption, retention, and separation of PFOA under low-pressure conditions. Specifically, the method includes the following steps: (1) Add iron salt coagulant to the water sample and then stir to form flocs; (2) Let the flocs generated in step (1) stand still and let them settle naturally on the surface of the ultrafiltration membrane to form a pre-deposited floc layer; (3) Add perfluorooctanoic acid solution to the water sample system containing the pre-deposited floc layer, allow it to stand and diffuse, and then perform ultrafiltration separation under low pressure to remove perfluorooctanoic acid.

[0009] The iron salt coagulant described in this invention is an FeCl3 solution, with a concentration of Fe... 3+ count.

[0010] Preferably, the dosage of iron salt coagulant in the water sample is 1~20 mg Fe / L, for example, it can be 2 mg Fe / L, 4 mg Fe / L, 6 mg Fe / L, 8 mg Fe / L, 10 mg Fe / L, 12 mg Fe / L, 14 mg Fe / L, 16 mg Fe / L, 18 mg Fe / L or 20 mg Fe / L, preferably 1 mg Fe / L, 2.5 mg Fe / L, 5 mg Fe / L, 10 mg Fe / L, 15 mg Fe / L and 20 mg Fe / L. However, it is not limited to the listed numbers, and other unlisted values ​​within the above range are also applicable.

[0011] This invention uses different dosages of iron salt coagulant to generate flocs because the structure and performance of the floc pre-deposition layer are closely related to the coagulant dosage. Specifically, the coagulant dosage directly affects the amount of flocs generated, as well as the pore structure and thickness of the floc layer, thus determining the number of adsorption sites and adsorption capacity of the floc layer for PFOA. Therefore, it is necessary to optimize the dosage to balance the relationship between removal efficiency and operating flux. A concentration range of 1~20 mg Fe / L is a commonly used dosage range for iron salt coagulants in the water treatment field. This dosage ensures effective coagulation without increasing sludge production or excessive treatment costs due to excessive dosage. Setting the process parameters of this invention within this range facilitates its integration with existing water treatment facilities and reduces the difficulty of promoting its engineering applications.

[0012] As a further preferred technical solution, the dosage of the iron salt coagulant is 5–10 mg Fe / L. Within this dosage range, the formed floc layer has a suitable thickness and pore structure, which can provide sufficient adsorption sites without excessively increasing filtration resistance, thereby achieving an optimal balance between removal efficiency and water flux.

[0013] The stirring method described in this invention includes fast stirring and slow stirring. The fast stirring speed is 150 rpm to 200 rpm, for example, 150 rpm, 160 rpm, 170 rpm, 180 rpm, 190 rpm, or 200 rpm. The slow stirring speed is 30 to 60 rpm, for example, 30 rpm, 40 rpm, 50 rpm, or 60 rpm, but is not limited to the listed numbers. Other unlisted values ​​within the above range are also applicable.

[0014] The floc settling method described in this invention is static natural settling.

[0015] Preferably, the settling time is 20-40 minutes, for example, 20 minutes, 25 minutes, 30 minutes, 35 minutes or 40 minutes, but it is not limited to the listed numbers. Other unlisted values ​​within the above range are also applicable.

[0016] The ultrafiltration membrane material described in this invention is polyvinylidene fluoride (PVDF) with a molecular weight cutoff of 100 kDa; As a preferred embodiment of the present invention, the ultrafiltration membrane needs to be pretreated before the floc pre-deposition. The ultrafiltration membrane pretreatment substrate operation is as follows: the brand-new ultrafiltration membrane is soaked in ultrapure water, and the water is changed several times during the soaking period to thoroughly remove impurities on the membrane surface.

[0017] Preferably, the soaking time is 12-24 hours, and the water is changed at least 3 times. For example, the soaking time can be 12 hours, 14 hours, 16 hours, 18 hours, 20 hours, 22 hours, or 24 hours, and the water can be changed 3 times, 4 times, or 5 times. However, it is not limited to the listed values, and other unlisted values ​​within the above range are also applicable.

[0018] In this invention, the pretreated ultrafiltration membrane is correctly installed at the bottom of the ultrafiltration cup. 300 mL of deionized water is added to the ultrafiltration cup. Then, using a pipette, a 0.1 M HCl or NaOH solution is added to finely adjust the pH value of the system to the experimental set value. Subsequently, 3 mL of a 0.1 M NaHCO3 solution is added as a buffer to maintain relative pH stability during the reaction.

[0019] As a preferred technical solution of the present invention, the pH value is 5.0~9.0, for example, it can be 5.0, 6.0, 7.0, 8.0 or 9.0, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0020] Preferably, the ultrafiltration process of the present invention is carried out at a gas pressure of 0.01 to 0.15 MPa. For example, it can be 0.01 MPa, 0.03 MPa, 0.05 MPa, 0.07 MPa, 0.09 MPa, 0.11 MPa, 0.13 MPa or 0.15 MPa, but is not limited to the listed values. Other unlisted values ​​within the above range are also applicable.

[0021] Preferably, the gas pressure is maintained by an inert gas, such as nitrogen, argon or helium, but not limited to the listed gases; other inert gases not listed above are also applicable.

[0022] The method of this invention also includes the determination of PFOA concentration in the effluent sample. The concentration of PFOA in the influent and effluent is determined using high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS / MS), and the PFOA removal rate is calculated according to the following formula: R = (C0 - C) p ) / C0× 100%; Among them, C0 and C p These represent the PFOA influent and effluent concentrations, respectively.

[0023] As a preferred embodiment of the present invention, the method selects HA or BSA as coexisting organic pollutants of PFOA.

[0024] This invention selects HA and BSA as coexisting organic substances for PFOA, taking into account both the complexity of actual water bodies and the needs of mechanism research. This allows for a comprehensive verification of the removal efficiency and mechanism of action of the process for PFOA under complex water quality conditions. On the one hand, the coexisting substances may compete with PFOA for adsorption sites on the floc surface, resulting in competitive adsorption. On the other hand, the coexisting substances themselves can be adsorbed by the flocs, forming organic-inorganic composite flocs, which may change the structure and surface properties of the floc layer, thereby affecting the adsorption behavior of PFOA.

[0025] The PFOA stock solution used in the water sample described in this invention is prepared as follows: Accurately weigh 1 mg of perfluorooctanoic acid (PFOA) standard powder and place it in a 100 mL brown volumetric flask. Add a small amount of methanol (analytical grade) to aid dissolution. After complete dissolution, dilute to the mark with ultrapure water to prepare a PFOA stock solution with a concentration of 10 mg / L. Transfer the solution to a brown reagent bottle and store it in a refrigerator at 4°C, protected from light, for later use.

[0026] The HA stock solution used in the water sample described in this invention is prepared as follows: 2 g of sodium humate (HA) powder is accurately weighed and slowly added to a beaker containing 1 L of ultrapure water. The beaker is placed on a magnetic stirrer and stirred continuously at 1000 rpm for 24 hours to ensure complete hydration and dissolution. The dissolved solution is then filtered through a 0.45 μm filter membrane, and the filtrate is collected and stored at 4°C in the dark to simulate the background composition of natural organic matter in the water.

[0027] The BSA stock solution used in the water sample described in this invention is prepared as follows: 200 mg of bovine serum albumin (BSA) powder is accurately weighed and added to 100 mL of ultrapure water. The solution is gently stirred at 500 rpm for 2 hours until completely dissolved. After dissolution, the solution is filtered through a 0.45 μm filter membrane to remove impurities. This stock solution must be freshly prepared on the day of the experiment to simulate the presence of protein-based organic pollutants.

[0028] In a second aspect, the present invention provides a floc pre-deposition-ultrafiltration combined treatment system, which is constructed using the method described in the first aspect.

[0029] The treatment system provided by this invention forms a stable floc-ultrafiltration membrane composite separation system by pre-depositing an iron salt floc layer on the surface of the ultrafiltration membrane. This system operates under low pressure and achieves a PFOA removal rate of up to 57.6%, thus realizing highly efficient removal of perfluorooctanoic acid (PFOA) from water. The system has a simple structure, low operating pressure, low energy consumption, and good adaptability to coexisting organic matter. It also exhibits good stability and reliability during long-term operation.

[0030] Thirdly, the present invention provides an application of the method described in the first aspect in drinking water treatment, advanced treatment of industrial wastewater, or surface water remediation.

[0031] The floc pre-deposition-ultrafiltration combined process described in this invention can be used for the control of perfluorinated new pollutants in various fields such as drinking water safety assurance, industrial wastewater discharge compliance, and pollution water environment treatment. It provides a feasible solution for developing low-energy-consumption and high-efficiency PFAS new pollutant removal technology, and has good application prospects and promotion value.

[0032] Compared with the prior art, the present invention has at least the following beneficial effects: This invention employs a combination of iron salt floc pre-deposition and low-pressure ultrafiltration, operating under nitrogen low-pressure conditions of 0.1 MPa. This significantly reduces energy consumption and operating costs, effectively avoiding the drawbacks of high-pressure membrane technologies such as nanofiltration and reverse osmosis, including high energy consumption and difficulty in treating concentrates. Experimental results show that this combined process achieves a significant leap in PFOA removal efficiency. Compared to the 7.6% rejection rate of ultrafiltration alone, this invention achieves a removal rate of 30.1% under conventional conditions, and up to 57.6% under optimized conditions (pH 5, FeCl3 dosage of 10 mg Fe / L, and coexistence of BSA), realizing highly efficient PFOA removal. Furthermore, this process exhibits good adaptability and synergistic removal effects for typical coexisting organic compounds in water, such as humic acid and bovine serum albumin, further demonstrating its potential application under complex water quality conditions. This invention not only provides a new approach for the enhanced removal of perfluorinated pollutants but also provides a feasible solution and theoretical basis for developing low-energy, high-efficiency removal technologies. Attached Figure Description

[0033] Figure 1 This is a diagram of the experimental apparatus for the present invention; Figure 2 A schematic diagram of the floc pre-deposition-ultrafiltration combined process for removing PFOA adsorbates provided by the present invention; Figure 3 This is a comparison of the PFOA removal effects of the methods provided in Examples 1-6 of the present invention; Figure 4 This is a comparison of the PFOA removal effects of the methods provided in Embodiments 7-18 of the present invention; Figure 5 This is a comparison of the PFOA removal effects of the methods provided in Examples 11-13 and 19-22 of the present invention, namely, a comparison of the PFOA removal effects of the floc pre-deposition-ultrafiltration combined process under different pH conditions. Figure 6 This is a comparison of the effects of Comparative Examples 1, 2, and 3 of the present invention with Example 1, that is, a comparison of the removal effects of different processes on PFOA. Detailed Implementation

[0034] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0035] Example 1 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration, the method comprising the following steps: (1) Place the pretreated PVDF ultrafiltration membrane (100 kDa) in an ultrafiltration cup; add 300 mL of deionized water to the ultrafiltration cup, adjust the pH to 7.0 with 0.1 M hydrochloric acid or sodium hydroxide, and add 3 mL of 0.1 M NaHCO3 buffer solution to maintain pH stability; (2) Add FeCl3 coagulant to make Fe 3+ The final concentration was 5 mg / L.

[0036] (3) Stir quickly at 200 rpm for 2 min, then stir slowly at 50 rpm for 15 min, and let stand for 20 min to allow the flocs to settle naturally on the surface of the ultrafilm. (4) Slowly add 1.5 mL of PFOA stock solution to the ultrafiltration cup to make the final concentration of PFOA 50 μg / L, let it stand for 30 min to allow it to diffuse fully, and take a sample after standing as the influent water sample; (5) Ultrafiltration was performed under nitrogen pressure of 0.1 MPa, and the filtrate was collected as the effluent sample; (6) Measure the PFOA concentration in the influent and effluent water samples and calculate the removal rate.

[0037] See appendix Figure 3 The experimental results show that the removal rate of PFOA in this embodiment is 30.1%.

[0038] Example 2 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration, which differs from Example 1 only in that FeCl3 coagulant is added in step (2) to make Fe 3+ The final concentration was 1 mg / L.

[0039] See appendix Figure 3 The experimental results show that the removal rate of PFOA in this embodiment is 21.0%.

[0040] Example 3 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration, the only difference from Example 1 being that FeCl3 solution is added in step (2) to make Fe 3+ The final concentration was 2.5 mg / L.

[0041] See appendix Figure 3 The experimental results show that the removal rate of PFOA in this embodiment is 26.8%.

[0042] Example 4 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration, which differs from Example 1 only in that FeCl3 coagulant is added in step (2) to make Fe3+ The final concentration was 10 mg / L.

[0043] See appendix Figure 3 The experimental results show that the removal rate of PFOA in this embodiment is 30.2%.

[0044] Example 5 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration, which differs from Example 1 only in that FeCl3 coagulant is added in step (2) to make Fe 3+ The final concentration was 15 mg / L.

[0045] See appendix Figure 3 The experimental results show that the removal rate of PFOA in this embodiment is 24.6%.

[0046] Example 6 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration, which differs from Example 1 only in that FeCl3 coagulant is added in step (2) to make Fe 3+ The final concentration was 20 mg / L.

[0047] See appendix Figure 3 The experimental results show that the removal rate of PFOA in this embodiment is 20.1%.

[0048] Example 7 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration, the difference between this method and Example 1 being: (1) After step (4) in Example 1, add HA stock solution to the simulated water sample to make its final concentration 20 mg / L, and let it stand for 30 minutes to allow it to diffuse fully; (2) Set the FeCl3 dosage to 1 mg Fe / L.

[0049] See appendix Figure 4 The experimental results show that the removal rate of PFOA in this embodiment is 20.1%.

[0050] Examples 8-18 A method for removing PFOA using floc pre-deposition-ultrafiltration, wherein the method steps differ from those in Example 7 only in the amount of FeCl3 coagulant added or the amount of coexisting organic matter.

[0051] For the PFOA removal rates of the methods provided in Examples 8-18, please refer to the appendix. Figure 4 .

[0052] Example 19 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration. The only difference between this method and Example 12 is that the pH of the simulated water sample is adjusted to 5.0.

[0053] See appendix Figure 5 The experimental results show that the removal rate of PFOA in this embodiment is 51.1%.

[0054] Example 20 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration. The only difference between this method and Example 14 is that the pH of the simulated water sample is adjusted to 5.0.

[0055] See appendix Figure 5 The experimental results show that the removal rate of PFOA in this embodiment is 57.6%.

[0056] Example 21 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration. The only difference between this method and Example 12 is that the pH of the simulated water sample is adjusted to 9.0.

[0057] See appendix Figure 5 The experimental results show that the removal rate of PFOA in this embodiment is 26.7%.

[0058] Example 22 This embodiment provides a method for removing PFOA using floc pre-deposition-ultrafiltration. The only difference between this method and Example 14 is that the pH of the simulated water sample is adjusted to 9.0.

[0059] See appendix Figure 5 The experimental results show that the removal rate of PFOA in this embodiment is 27.6%.

[0060] Comparative Example 1 This comparative example provides a method for removing PFOA by ultrafiltration (UF) alone, which specifically includes the following steps: (1) Install the pretreated PVDF ultrafiltration membrane (100 kDa) into the ultrafiltration cup and add 300 mL of deionized water into the ultrafiltration cup; (2) Add 1.5 mL of PFOA stock solution to make the final concentration of PFOA 50 μg / L, let stand for 30 min to allow PFOA to diffuse fully, and take a sample after standing as the influent water sample.

[0061] (3) Ultrafiltration was performed under a nitrogen pressure of 0.1 MPa, and the effluent water sample was collected.

[0062] (4) Measure the PFOA concentration in the influent and effluent and calculate the removal rate.

[0063] See appendix Figure 6 The experimental results showed that the removal rate of PFOA in this comparative sample was 7.6%.

[0064] Comparative Example 2 This comparative example provides a method for removing PFOA (Coa) through coagulation alone, which specifically includes the following steps: (1) Add 300 mL of deionized water to the ultrafiltration cup, and simultaneously add FeCl3 solution to make Fe... 3+ The final concentration was 5 mg / L; (2) Add 1.5 mL of PFOA stock solution to make the final concentration of PFOA 50 μg / L, and take the water sample at this time as the influent water sample; (3) Stir quickly at 200 rpm for 2 min, then stir slowly at 50 rpm for 15 min.

[0065] (4) Take the water sample after stirring as the effluent sample, measure the PFOA concentration in the influent and effluent, and calculate the removal rate.

[0066] See appendix Figure 6 The experimental results showed that the removal rate of PFOA in this comparative sample was 18.4%.

[0067] Comparative Example 3 This comparative example provides a method for removing PFOA from ultrafiltration supernatant after coagulation and sedimentation (CS-UF), which specifically includes the following steps: (1) After completing steps (1) to (4) in Comparative Example 2, let the solution stand for 20 min, take the supernatant and transfer it to an ultrafiltration cup containing a pretreated ultrafiltration membrane and ultrafilter it under a nitrogen pressure of 0.1 MPa. Collect the filtrate as an effluent water sample.

[0068] (2) Measure the PFOA concentration in the influent and effluent and calculate the removal rate.

[0069] See appendix Figure 6 The experimental results showed that the removal rate of PFOA in this comparative sample was 23.7%.

[0070] Comparative Example 4 This comparative example provides a method for removing PFOA using a combination of coagulation and ultrafiltration (C-UF). The only difference between this method and Comparative Example 3 is that the solution after coagulation is not allowed to stand for precipitation; instead, ultrafiltration is performed immediately. Otherwise, the methods are the same as those in Comparative Example 3.

[0071] See appendix Figure 6 The experimental results showed that the removal rate of PFOA in this comparative sample was 22.0%.

[0072] (1) As can be seen from Examples 1 to 6, the present invention, by employing a process combining iron salt floc pre-deposition and low-pressure ultrafiltration, pre-constructs a floc deposition layer on the surface of the ultrafiltration membrane, forming a "floc layer-ultrafiltration membrane" composite separation system, and achieves effective removal of perfluorooctanoic acid (PFOA) from water under low-pressure operating conditions of 0.1 MPa. When the FeCl3 dosage is 5 mg / L (Example 1), the removal rate of PFOA can reach 30.1%; when the FeCl3 dosage is 1–20 mg / L (Examples 1–6), the removal rate of PFOA shows a trend of first increasing and then decreasing with the increase of dosage, and the best removal effect is achieved in the dosage range of 5–10 mg / L.

[0073] (2) As can be seen from Examples 7 to 19, the present invention can achieve better technical effects by further optimizing process parameters and controlling coexisting pollutants: When 20 mg / L of humic acid coexisting organic matter such as HA is present in the water, the removal rate of PFOA is about 28.5% under the condition of 5 mg Fe / L dosage (Example 11); when 20 mg / L of protein coexisting organic matter such as BSA is present in the water, the removal rate of PFOA can be increased to 43.9% under the condition of 5 mg Fe / L dosage (Example 12); when the pH of the system is optimized to acidic conditions (pH 5.0) and the FeCl3 dosage is 10 mg / L, and the coexisting organic matter is 20 mg / L BSA (Example 20), the removal rate of PFOA can reach up to 57.6%; while when the pH of the system is alkaline conditions (pH 9.0) (Example 22), the removal rate drops to below 30%, indicating that acidic conditions are conducive to improving the removal effect of the present invention.

[0074] (3) By comparing Example 1 with Comparative Examples 1 to 4, it can be seen that the present invention can achieve a synergistic effect by using a combination of floc pre-deposition and ultrafiltration: compared with the 7.6% rejection rate of ultrafiltration alone (Comparative Example 1), the 18.4% removal rate of coagulation alone (Comparative Example 2), the 23.7% removal rate of ultrafiltration of supernatant after coagulation and sedimentation (Comparative Example 3), and the 22.0% removal rate of direct coagulation-ultrafiltration (Comparative Example 4), the floc pre-deposition-ultrafiltration combination process of Example 1 of the present invention can increase the removal rate to 30.1%. However, when the pre-deposition step of flocs on the membrane surface is missing (as shown in Comparative Example 4) or when flocs are not deposited on the membrane surface and only the supernatant is ultrafiltered (as shown in Comparative Example 3), the synergistic removal effect of the present invention cannot be achieved, and the removal rate is significantly lower than that of the embodiments of the present invention.

Claims

1. A method for removing perfluorooctanoic acid (PFOA) from water using a pre-deposited flocculent layer-ultrafiltration combined process, characterized in that, Includes the following steps: (1) Add iron salt coagulant to the water sample and then stir to form flocs; (2) Let the flocs generated in step (1) stand still and let them settle naturally on the surface of the ultrafiltration membrane to form a pre-deposited floc layer; (3) Add perfluorooctanoic acid solution to the water sample system containing the pre-deposited floc layer, allow it to stand and diffuse, and then perform ultrafiltration separation under low pressure to remove perfluorooctanoic acid.

2. The method according to claim 1, characterized in that, The iron salt coagulant is FeCl3.

3. The method according to claim 2, characterized in that, The dosage of iron salt coagulant in the water sample was 1~20 mgFe / L; Preferably, the dosage of the iron salt coagulant is 5-10 mg Fe / L.

4. The method according to any one of claims 1-3, characterized in that, The mixing process includes rapid mixing at 200 rpm for 2 minutes and slow mixing at 50 rpm for 15 minutes.

5. The method according to any one of claims 1-4, characterized in that, The natural settling time of the flocs is 20 minutes; The static diffusion time is 30 min; The low-pressure condition is 0.1 MPa; The ultrafiltration membrane is made of polyvinylidene fluoride (PVDF) with a molecular weight cutoff of 100 kDa. The initial concentration of perfluorooctanoic acid (PFOA) in the water sample was 50 μg / L.

6. The method according to claims 1-5, characterized in that, NaHCO3 was added to the water sample as a buffer solution to maintain pH stability.

7. The method according to any one of claims 1-6, characterized in that, Add coexisting organic matter to the water sample; Preferably, the coexisting organic matter is humic acid or bovine serum albumin; Preferably, the concentration of the coexisting organic matter is 20 mg / L.

8. The method according to any one of claims 1-7, characterized in that, The pH of the water sample was adjusted using 0.1 M hydrochloric acid or sodium hydroxide. Preferably, the pH adjustment is 5.0 to 9.

0.

9. A floc pre-deposition-ultrafiltration combined process according to claims 1-8, used to remove perfluorooctanoic acid from water.

10. A method for removing perfluorooctanoic acid from water using the floc pre-deposition-ultrafiltration combined process as described in claims 1-9, which can be applied to the control of perfluorinated new pollutants in various fields such as water treatment, industrial wastewater discharge compliance, and environmental remediation of polluted water bodies.

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